1. Field of the Invention
This invention relates to a mixer circuit for use with a high frequency appliance such as a television tuner or a cable television converter.
2. Description of Related Art
Mixer circuits are conventionally known and employed widely in various high frequency appliances. An exemplary one of such conventional mixer circuits is shown in FIG. 2. Referring to FIG. 2, the conventional mixer circuit shown employs an emitter-coupled amplifier circuit and includes a double balanced mixer constituted from transistors Q.sub.1 to Q.sub.4 and a differential amplifier constituted from transistors Q.sub.5 and Q.sub.6.
The double balanced mixer is constituted from a differential circuit of the grounded-base type wherein the emitters of the transistors Q.sub.1 and Q.sub.2 are connected commonly by way of a common emitter terminal 1 and another differential circuit of the grounded-base type wherein the emitters of the transistors Q.sub.3 and Q.sub.4 are connected commonly by way of another common emitter terminal 2, and the bases of the transistors Q.sub.1 and Q.sub.4 are connected to an input terminal A of the double balanced mixer while the bases of the transistors Q.sub.2 and Q.sub.3 are connected to another input terminal B of the double balanced mixer. A local oscillation signal is supplied to the input terminals A and B of the double balanced mixer. Meanwhile, the collectors of the transistors Q.sub.1 and Q.sub.3 are connected to a common collector terminal C while the collectors of the transistors Q.sub.2 and Q.sub.4 are connected to another common collector terminal D, and a power terminal Vcc is connected to the common collector terminals C and D by way of impedance terminals R.sub.1 and R.sub.2, respectively, while output terminals G and H of the double balanced mixer are connected to the common collector terminals C and D.
On the other hand, the differential amplifier is constituted such that the bases of the transistors Q.sub.5 and Q.sub.6 are connected to a pair of input terminals E and F of the differential amplifier, respectively, while a pair of impedance elements R.sub.3 and R.sub.4 connected in series are connected between the emitters of the transistors Q.sub.5 and Q.sub.6, and a junction between the impedance elements R.sub.3 and R.sub.4 is grounded by way of a current source I.
With such construction, if a high frequency signal is supplied to the input terminals E and F of the differential amplifier by way of a band-pass filter (BPF), then it is amplified by the differential amplifier constituted from the transistors Q.sub.5 and Q.sub.6, and the thus amplified high frequency signal is outputted from the collectors of the transistors Q.sub.5 and Q.sub.6. The signal outputted from the collector of the transistor Q.sub.5 is supplied to the common emitter terminal 1 of the double balanced mixer while the signal outputted from the collector of the transistor Q.sub.6 is supplied to the other common emitter terminal 2 of the double balanced mixer. Then, as a local oscillation signal is inputted to the input terminals A and B, a mixed signal of the high frequency signal and the local oscillation signal is produced the double balanced mixer due to its non-linear operation and outputted from the double balanced mixer by way the output terminals G and H.
The conventional mixer circuit described above has such drawbacks as follows. In particular, while the differential circuits constituted from the transistors Q.sub.1 and Q.sub.2 and the transistors Q.sub.3 and Q.sub.4 and constituting the double balanced mixer are of the grounded-base type, generally a differential circuit of such grounded-based type is very low in input impedance, and accordingly, the load impedance of the differential amplifier to which the differential circuits are connected as a load is very low. Consequently, the differential amplifier does not have a desirable distortion characteristic, and deterioration in distortion characteristic such as cross modulation or intermodulation of the entire mixer circuit cannot be eliminate. This is apparent from simulation by non-linear software (microwave harmonica) of the Compact Company of the United States.
Further, while a high frequency signal is inputted to the differential amplifier by way of a band-pass filter, since the load impedance of the differential amplifier is extremely low, the input impedance of the differential amplifier presents a negative resistance and serves as a load to the band-pass filter. Accordingly, such problem takes place that the flatness in the band of the band-pass filter is deteriorated or the band width is reduced, and particularly since recent transistors are high in gain band width product Ft, this problem appears remarkably.
Furthermore, since the input impedance of the differential amplifier presents a negative resistance as described above, it is difficult to assure impedance matching between the differential amplifier and the band-pass filter, and if, for example, a resistor is added in order to eliminate the negative resistance, then another new problem takes place that the transmission loss is increased and the NF (noise figure) is deteriorated.
In addition, since the load impedance of the differential amplifier is low, the transistors Q.sub.5 and Q.sub.6 form an oscillator of the grounded-collector type. Accordingly, abnormal oscillations are likely produced.